A double seat valve of the generic type, which is also designated as a double seat valve with cleanable seats due to its function, to allow cleaning of the respective seat exposed by gap-wide opening of its seats, is known from DE 196 08 792 C2. Each of the three seals in the known double seat valve is a discrete seal, wherein each seal only has one function. The first seal in the first closing element is a purely radially acting seal, which slides in the first seat formed as a cylindrical envelope and seals there as a result of the provided bias (seal with sliding engagement). The second seal in the second closing element either acts upon the conical formed second seat such that this can be designated as an axially/radially acting seal (seal with pressing engagement), or a purely radially acting seal is provided here as well (seal with sliding engagement). As a rule, the fixed (metal) stop for limiting the final position of the second closing element (closing position) is also located at the second seat, if a seal with pressing engagement is provided. The third seal, the so-called intermediate seal, deploys its effect, when the first closing element comes to abut at the second closing element through this intermediate seal during the opening lift and carries the second closing element along into the completely open position in the course of the further opening movement (seal with pressing engagement). In this, the intermediate seal is arranged in a front face directed towards the leakage cavity at the second, dependently driven closing element.
The known double seat valve outlined above in short comprises two so-called seat cleaning positions, wherein the first seat cleaning position is created by a first partial lift opposed to the opening lift. In this position the cylindrical lug arranged at the first closing element on the side of the leakage cavity forms a so-called choke gap with the associated cylindrical first seat, through which the amount of cleaning agent supplied from the adjacent first valve housing part can be restricted. The seat cleaning position of the second closing element occurs by a partial lift in the same direction as the opening lift, wherein in the partially open position the cylindrical lug arranged at the second closing element on the side of the leakage cavity forms a second choke gap with the associated part of the connecting opening, which limits the amount of the seat cleaning flow created in this seat cleaning position.
Since the two closing elements and the associated cylindrical lugs comprise different diameters, the respective associated sections of the connecting opening are also of different diameter such that a transition area is obtained between these two diameters. In the seat cleaning position of the first closing element the first seat cleaning flow flows along the cylindrical seat and hits the frontal boundary of the second closing element, wherein it is prevented by the difference in diameter of the two sections of the connecting opening that the first seal cleaning flow directly hits the second choke gap and thus the seat area of the second closing element. In the seat cleaning position of the second closing element, the second seat cleaning flow is diverted radially inwards at the transition area bridging the difference in diameter between the sections of the connecting opening and is guided past the frontal boundary surface of the first closing element. Also in this case it is prevented that the second seat cleaning flow directly hits the first choke gap and thus the seat area of the first closing element.
With the known double seat valve it is possible to limit the amount of cleaning agent used in the respective seat cleaning as desired. However, the requirements to be satisfied by such a double seat valve in certain countries go beyond this. Thus it is demanded in the USA, for example, that in case of larger sealing deficiencies or even the loss of one of the two seat seals in the course of the seat cleaning of the other closing element, no cleaning agent may penetrate through the respective sealing deficiency or the seat area without seat seal, respectively. Under these conditions such a double seat valve is not only subject to the requirement of a restriction of the amount of cleaning agent and the avoidance of a direct admission of the seat areas in the course of the seat cleaning, but also of a discharge possibly free of turbulence of the seat cleaning flow into the leakage cavity at first and into the surrounding from there, without the respective closed seat area being directly hit by this seat cleaning flow or acted upon increasing pressure thereon.
Direct admission means each velocity component from the respective seat cleaning flow directed vertically onto the walls defining the seat area. Namely, it has been noted that each direct admission in this regards results in a conversion of kinetic energy of flow to static pressure. Depending on the impact angle of the flow onto the wall or body surface flown against, a branching flow with a so-called “branching flow line” is obtained, the latter parting the flow into two halves. The branching flow line itself runs against the so-called “stagnation point”, such that the velocity is zero at this point. The pressure increase as a result of this stopping of the velocity is also designated as “impact pressure”. The above illustrated mechanisms increasing pressure generate a leakage flow across the respective choke gap and the damaged or completely missing seat seal.
Therefore, direct hitting by the seat cleaning flow onto the surface defining the leakage cavity is counterproductive in any case. In the known double seat valve, the first seat cleaning flow, which is generated by lifting the first closing element by the first partial lift, can more or less perpendicularly impact onto the frontal boundary area of the second closing element, which accommodates the intermediate seal. At the place of impact this flow is diverted mainly to the centre of the leakage cavity and the flow i. a. also affects the intermediate seal projecting from the front face. Furthermore, a branching flow line is obtained at the place of impact, the branch of which directed towards the seat area can generate a vortex and an impact pressure there.
Further double seat valves are known (WO 98 54 494 A1; EP 0 819 876 B1), which dispense with the intermediate seal indicated above. In the double seat valve according to WO 98 54 494 A1 the radially acting first seat seal of the first closing element is driven in the course of the opening movement into a cylindrical recess of the same diameter in the second closing element, which is aligned with the cylindrical seat of the first closing element. Only then the second closing element is lifted from its seat by the first closing element and both closing elements together reach the completely open position. In a configuration in this regard the radial seal of the first closing element takes on the sealing in the closed position and also the function of the so-called intermediate seal in the collecting position and the following open position of the double seat valve, wherein the first seat seal acts purely radially (sliding engagement) also in these positions. The second closing element is formed as a seat disk, the seal of which co-operates with a conical section of the second seat, and it is also guided and centred at this conical section. If the seal allows it, a fixed (metal) stop for limiting the final position of the second closing element in the closed position of the double seat valve is provided at the conical section.
In the seat cleaning of the first closing element, the seat cleaning flow is guided along the cylindrical wall of the first seat and the following recess and impacts against the frontal boundary of the cylindrical recess, which is preferably oriented vertically to the flush cylindrical faces. Such a right-angled diversion of the flow is unfavourable as a whole, it detaches uncontrollably from the wall and creates vortexes and circulation flows, which can form impact pressure. The known configuration has a further drawback that the shape of the recess in the second closing element is determined by the shape and dimension of the cylindrical seat in the valve housing. For this reason, there are only restricted possibilities for the fluidic design of this recess guiding and steering the seat cleaning flow.
In EP 0 819 876 B1 a double seat valve with cleanable seats is described, which is also implemented without an intermediate seal and wherein the first seat seal radially acting in the cylindrical valve seat has a double function. This double function consists in that the first seat seal arranged on the first closing element formed as a slide piston comprises simultaneously to a radial sealing function within the cylindrical valve seat an axial sealing function with regard to the second closing element (pressing engagement). This axial sealing function becomes effective again, when the first closing element in the course of the opening movement reaches a so-called collecting position with the second closing element and transfers this into an open position as well then.
As opposed to the double seat valve according to DE 196 08 792 C2 and the double seat valve according to WO 98 54 494-A1, in the double seat valve according to EP 0 819 876 B1 the choke gap at the first closing element is not arranged on the side of the leakage cavity, but on the side averted from the leakage cavity. The cylindrical lug for forming the possible choke gap with the associated surface of the connecting opening is located, with regard to the illustrated position, below the first closing element. In order to remove in this configuration the radially acting first seat seal gap-wide from its associated cylindrical valve seat, an annular recess in the connecting opening is required, into which the first seat seal is driven in the course of the associated seat cleaning. The cylindrical lug at the first closing element forms the desired choke gap with an associated section of the connecting opening below this annular recess then. Regarding the sole choke gap effect, the rearrangement of the choke gap from a position on the side of the leakage cavity to one averted from the leakage cavity may be equivalent, however, a situation critical regarding cleaning for the first seat seal in the first closing element arises from this modification, when the first closing element is transferred to its seat cleaning position. In this case there is the risk that at least partial amounts of the second seat cleaning flow impact directly onto the sealing area of the first closing element, because the cylindrical lug forming the choke gap is missing, which is usually arranged in front of and shields this first seat seal.
Seat seals of double seat valves comprising double functions in the manner described above are not without problems, as they have to satisfy different requirements and their sphere of action is not clearly defined and limited, as it is the case with discrete seals each comprising an exclusively radial or exclusively axial/radial or exclusively axial function. Due to the fact that a single seal acts purely radially on the one hand and must assume in a more or less directly adjacent area axial or axial/radial, respectively, sealing functions, these respective sealing areas have to be brought together relatively closely in order to avoid that the seal will obtain unnecessary large dimensions. In the present case the purely radially acting sealing area must remain that long in the cylindrical seat, until the axially/radially acting area has come to abut against the other closing element. The part of the seat seal with double function bridging the two sealing areas indicated above is as a ruled distanced from the cylindrical seat such that between seal and cylindrical seat a sump-like, deep notch arises, which presents an area critical with regard to cleaning.
It is the object of the present invention to further develop a double seat valve of the generic type such that a discharge possibly free of turbulence of the seat cleaning flow into and out of the leakage cavity is ensured and a direct admission of the seat area is definitely avoided.